This invention concerns gas turbine engines and, more particularly but not exclusively, gas turbine engines in which steam is injected and a method of operating such engines.
Steam injection is employed on industrial engines to reduce NOx emissions and boost power. The specific fuel consumption of the engine can also be improved as the steam is usually raised using engine exhaust heat, in a heat recovery steam generator (HRSG).
In one known system, described in U.S. Pat. No. 4,569,195, a steam injection gas turbine has an intercooler situated between a low-pressure compressor and a high pressure compressor. Water is used as the coolant in the intercooler to reduce the temperature of the air before it enters the high pressure compressor thereby reducing the volumetric flow rate entering, and consequently the work required to drive, the high pressure compressor. More power can thus be extracted from the power turbine. The HRSG takes water from the intercooler and generates steam that may be injected to various parts of the gas turbine, including the combustor and the turbines. The steam injection increases the mass flow and the enthalpy of the working fluid passing through the turbines, resulting in further power extraction from the turbines.
Generally, the flow rate through the intercooler necessary to cool the air flow to the high pressure compressor is much higher than the steam injection flow rate. For the system described in U.S. Pat. No. 4,569,195, in its practical implementations, excess water from the intercooler is dumped from the cycle or re-circulated after cooling in another large heat exchanger at the cost of reduced utilisation of the heat absorbed by that water in the intercooler and that heat is lost to the cycle. Reducing the flow rate of water through the intercooler to match that of the rate of steam injection does not always contribute to the improvement of the overall thermal efficiency or plant economics. Firstly, reducing the flow rate of the water through the intercooler is only possible within the range that the water temperature does not exceed the air temperature in the intercooler. Secondly, when the final steam temperature from the HRSG is fixed, with the hotter feed water temperature to the HRSG resulting from the reduced water flow rate at the intercooler the exhaust temperature at the exit of the HRSG increases. Namely, the intercooler just substitutes some part of water pre-heating in the HRSG without achieving any improvement in the efficiency. In order to improve the thermal efficiency by utilising the heat from the intercooler, the final steam temperature from the HRSG needs to be increased and the exhaust temperature at the exit of the HRSG needs to be maintained or decreased. To improve the overall efficiency with the hotter feed water temperature to the HRSG, in the case of the reduced water flow rate at the intercooler, the HRSG has to be much larger and more expensive because the hotter feed water temperature to the HRSG reduces the mean temperature difference between the exhaust side and water side of the HRSG and that determines HRSG sizing.
A further problem with injecting steam into the cycle of the gas turbine is water consumption because the steam injected into the cycle is in the end discharged to the atmosphere together with the exhaust unless it is condensed and recovered before the final exhaust. Whilst it is possible to condense the steam and recover most of the water from the exhaust, the exhaust after a condenser is saturated therefore the remaining steam within the low temperature exhaust is ejected to the atmosphere and is visible, under some circumstances, as a plume. The plume is often equated with pollution by the general public and can have a detrimental effect on public opinion. It is therefore desirable to suppress the plume formation.
In U.S. Pat. No. 6,845,738 water within the exhaust of a turbine is condensed, stored and re-used for injecting as steam upstream of the combustor. The heat exchanger as a HRSG upstream of the condenser has water on the secondary side. It is important that the water does not dry up, particularly during startup to ensure that the temperature of the expanded working fluid is kept at a permanently low level when it enters the condenser. Failure to keep the temperature low will result in damage to the condenser itself. When a conventional HRSG is employed, water must always be present in the heat exchanger and it is important that the turbine is started slowly to avoid thermal shock and damage to the HRSG.
In some gas turbine systems, steam is generated in a Once Through Steam Generator (OTSG) and used to power a steam turbine in addition to providing the necessary steam for steam injection. Such a system is described in U.S. Pat. No. 5,727,377. An OTSG does not have the thick walls of the drums of conventional HRSGs, which means that start-up times are generally short. The fast start-up time is achieved, in part, by the capability of the OTSG to run dry i.e. without any water flowing through the tubes on the secondary side of the heat exchanger.
Thus, an OTSG creates a problem unique over conventional HSRG systems, in that components downstream of the OTSG are subjected to gas that has not been cooled in any significant manner. Therefore, the exhaust of an OTSG is conventionally vented to atmosphere through the turbine without any downstream components.